1. Field of the Invention
The present invention relates to an x-ray mask for use in an x-ray lithography method and an x-ray mask blank which is part of the x-ray mask.
2. Description of the Related Art
In the semiconductor industry, the technique of forming an integrated circuit constituted of a fine pattern on a silicon substrate or the like using a photolithography method for transferring the fine pattern by the use of a visible light and an ultraviolet light as an exposing electromagnetic wave, is well known. However, a recent advance in semiconductor technique greatly promotes the high integration of a semiconductor device such as a VLSI. This results in a requirement for the technique for transferring the fine pattern of high accuracy beyond the transfer limit (a principled limit due to wavelength) of the visible light and the ultraviolet light used in the conventional photolithography method. In order to transfer such a fine pattern, an x-ray lithography method, using an x-ray whose wavelength is shorter than the wavelength of the visible light and the ultraviolet light, is attempted.
FIG. 1 is a cross sectional view showing a structure of an x-ray mask for use in the x-ray lithography. FIG. 2 is a cross sectional view showing the structure of an example of an x-ray mask blank as an intermediate product obtained in an intermediate step during manufacturing the x-ray mask.
As shown in FIG. 1, an x-ray mask 1 comprises an x-ray membrane 12 for transmitting the x-ray and an x-ray absorbing film pattern 13a formed on the x-ray membrane 12. The x-ray membrane 12 is supported by a silicon frame body 11a which is formed by removing a portion so that the periphery alone of the silicon substrate may remain. When this x-ray mask 1 is manufactured, the x-ray mask blank intermediate product is manufactured in the intermediate step. This x-ray mask blank is further processed, so that the x-ray mask is obtained. In this industry, while, the x-ray mask is a finished product which is dealt the x-ray mask blank, which is the intermediate product, is also often independently dealt.
As shown in FIG. 2, an x-ray mask blank 2 comprises the x-ray membrane 12 formed on a silicon substrate 11 and an x-ray absorbing film 13 formed on the x-ray membrane 12.
Silicon nitride, silicon carbide, diamond or the like is generally used as the x-ray membrane 12. An amorphous material, including tantalum (Ta), having an excellent resistance to x-ray radiation, is often used as the x-ray absorbing film 13.
One example of a process of manufacturing the x-ray mask 1 from the x-ray mask blank 2, is the following example. A resist film on which a desired pattern is formed is arranged on the x-ray mask blank 2 shown in FIG. 2. This pattern is then used as a mask so as to perform a dry etching, so that the x-ray absorbing film pattern is formed. After that, a center area, formed on a rear surface and to be a window area of the x-ray membrane 12, is removed by a reactive ion etching (RIE) using 4-fluorocarbon (CF4) as etching gas. The remaining film (12a: see FIG. 1) is then used as the mask to etch the silicon substrate 11 using an etching liquid constituted of a mixed liquid of fluoric acid and nitric acid, and whereby the x-ray mask 1 (see FIG. 1) is obtained. In this case, an electron beam (EB) resist is generally used as the resist, and the pattern is formed by means of an EB lithography.
For the x-ray membrane 12, a high transmittance to the x-ray, a high Youngis modulus of elasticity, a proper tensile stress, a resistance to x-ray radiation, the high transmittance within a visible light range, and the like are required. These characteristics will be described below. The transmittance to the x-ray is required during an exposure. The higher the transmittance is, the shorter the time required for the exposure. This is effective for improving throughput. The Youngis modulus of elasticity has an influence on strength of the film and deformation of an absorber pattern. The higher the Youngis modulus of elasticity is, the higher the film strength. This is effective for suppressing misalignment. The proper tensile stress is needed so that the film is self-supported. Since the x-ray membrane is irradiated with the x-ray during the exposure, it is necessary to not have damage due to this x-ray radiation, and thus the resistance to x-ray radiation is required. Since an alignment of the mask attached to an x-ray stepper and a wafer is accomplished by the use of a light source within the visible light range, high transmittance to an alignment light source (the visible light) is needed in order to achieve a highly accurate alignment. Furthermore, the film surface is required to be smooth. Surface smoothness is needed for highly accurate pattern formation on the absorber.
In order to satisfy these requirements, various materials and manufacturing methods have been studied. Since it is confirmed that the silicon carbide has damage due to the x-ray in the silicon nitride, the silicon carbide (SiC) and the diamond which have been heretofore used as the x-ray membrane, it may safely be said that the silicon carbide is the most promising material. However, since the SiC film generally has a polycrystalline structure, it has the film surface which is rougher than 6 nm (Ra: a center-line average roughness). To smooth the surface of this SiC film, an etch back method and a mechanical polishing method are carried out after the film formation. In etch back method, the rough SiC film is coated with the resist, and the thus obtained smooth resist surface is transferred onto SiC film by the dry etching. For mechanical polishing, a hard grain such as the diamond and alumina is used as an abrasive material so as to physically grind the unevenness on the surface of the SiC film. For example, according to Japanese Patent Publication No. 7-75219, a surface roughness of 20 nm or less is obtained by the etch back and the mechanical polishing. Although the definition of the surface roughness is not clear in this publication, this roughness is expected to be a maximum height (Rmax) and corresponds to about 2 nm or less in terms of Ra.
Recently, due to the advance in photolithography technique, introduction of the x-ray lithography has been performed later. At present, introduction from a generation of 1G bit-DRAM (design rule: 0.18 μm) is anticipated. Even if the x-ray lithography is introduced from 1G, the x-ray lithography is characterized in that it can be used through a plurality of generations up to 4G, 16G and 64G. Assuming that the x-ray lithography is used for 64G, the position precision required for the x-ray mask becomes severer, and is required to be as high as 10 nm. The inventor has already found that, in order to suppress such a position precision, it is effective to equalize an internal stress in an area in which a mask pattern is formed on the x-ray absorbing film (Japanese Patent Application No. 8-233402). As a result of a further study, the inventor has found that the surface roughness of the x-ray membrane has a sensitive influence on a stress distribution of the x-ray absorbing film formed on the x-ray membrane.